The technical field of this invention relates generally to alteration of apoptosis in a cell and modification of pathways involved in apoptosis including the treatment of diseases that involve apoptotic pathways. In particular, the invention pertains to a class of inhibitors that inhibit mitochondrial homeostasis, caspase-dependent, and caspase-independent apoptosis by affecting Omi/HtrA2 activity.
Apoptosis, which is also referred to as programmed cell death, is a form of cell death characterized by membrane blebbing and nuclear DNA fragmentation. Apoptotic cell death is morphologically distinct from necrotic cell death and is important in the normal development and maintenance of multicellular organisms.
Since apoptosis functions in maintaining tissue homeostasis in a range of physiological processes such as embryonic development, immune cell regulation and normal cellular turnover, the dysfunction or loss of regulated apoptosis can lead to a variety of pathological disease states. For example, the loss of apoptosis can lead to the pathological accumulation of self-reactive lymphocytes that occurs with many autoimmune diseases. Inappropriate loss or inhibition of apoptosis can also lead to the accumulation of virally infected cells and of hyperproliferative cells such as neoplastic or tumor cells. Similarly, the inappropriate activation of apoptosis can also contribute to a variety of pathological disease states including, for example, acquired immunodeficiency syndrome (AIDS), neurodegenerative diseases and ischemic injury. Treatments that are specifically designed to modulate the apoptotic pathways in these and other pathological conditions can alter the natural progression of many of these diseases.
Although apoptosis is mediated by diverse signals and complex interactions of cellular gene products, the results of these interactions ultimately feed into a cell death pathway that is evolutionarily conserved between humans and invertebrates. The pathway, itself, is a cascade of proteolytic events analogous to that of the blood coagulation cascade.
Several gene families and products that modulate the apoptotic process have now been identified. Key to the apoptotic program is a family of cysteine proteases termed caspases. The human caspase family includes Ced-3, human ICE (interleukin-1-beta converting enzyme) (caspase-1), ICH-1 (caspase-2), CPP32 (caspase-3), ICErelII (caspase-4), ICErelII (caspase-5), Mch2 (caspase-6), ICE-LAP3 (caspase-7), Mch5 (caspase-8), ICE-LAP6 (caspase-9), Mch4 (caspase-10), caspase 11-14, and others. Capsases can be positioned as downstream effectors of apoptosis.
The serine protease Omi, is also involved in apoptosis. Omi (also known as “HtrA2” and “Omi/HtrA2”) was identified as a mitochondrial direct inhibitor of apoptosis protein (IAP) binding protein, and is released from the mitochondria upon induction of apoptosis by apoptotic stimuli (Hegde et al. (2002) J. Biol. Chem., 277: 432-438). The mature Omi/HtrA2 protein contains a conserved IAP-binding motif that comprises the tetra peptide motif “AVPS” at its N terminus. It has been demonstrated that the deregulated expression of Omi/HtrA2 in the cytoplasm of mammalian cells induces apoptosis in these cells, indicating that Omi/HtrA2 could participate in the mitochondrial apoptotic pathway.
Mature Omi/HtrA2 is released from the mitochondria into the cytosol upon disruption of the outer mitochondrial membrane during apoptosis. Mature Omi/HtrA2 can induce apoptosis in human cells in a caspase-independent manner through its protease activity and in a caspase-dependent manner via its ability to disrupt caspase-IAP interaction.
Since Omi/HtrA2 is present in all mammalian cells and by its activity of disrupting the inhibition of the “apoptosis protein-caspase interaction” decreases the cell lifetime, it would be important to discover an inhibitor of the adverse activity of Omi/HtrA2 and homologous proteins.
Accordingly, it would be useful to identify a class of inhibitors that inhibit Omi/HtrA2. A need also exists to determine the peptides involved in the caspase-independent pathway of apoptosis such as substrates of Omi/HtrA2, as well as methods and compositions that inhibit such pathways. It would also be useful to find ways to use these methods and compositions to prevent, reduce or ameliorate a disorder associated with apoptosis.